At 9:30 p.m. tonight, Pacific Time, the Vernal Equinox occurs. Astronomers actually pin it down to 4:30 a.m UTC/GMT tomorrow in London, which is the same moment. See a Slate article for some technical detail.
Starting tomorrow, there will be marginally more minutes of light than of darkness. This condition will persist until roughly September 21 in the northern hemisphere.
Regardless of what the weather is doing where you are, the Vernal (Spring) Equinox is the generally accepted start of the season we call "Spring." It has been feeling like Spring for some time in NorCal, as is typical. The almond trees bloomed weeks ago, allergy season is upon us.
By contrast, our part of Wyoming is white and shiny under a fresh coating of snow, and likely will be so for some weeks at its over-a-mile-high elevation in the Rockies. The region rarely gets deep snow; its winters, while not especially hard, are long.
The growing season - between the last hard frost of Spring and the first hard frost of Fall - is less than 60 days. The most practical field crop is irrigated fodder - raising, cutting, and baling hay - which is fed to cattle and horses.
